A crow's nest is a structure in the upper part of the mainmast of a ship that is used as a lookout point.
This position ensured the best view of the approaching hazards, other ships or land. It was the best device for this purpose until the invention of radar. It was a sailor in the crow's nest (Rodrigo de Triana) who actually witnessed the Americas in 1492, rather than Christopher Columbus.
Since the crow's nest is a point far away from the ship's center of mass, any small movement of the ship is amplified and could lead to severe seasickness, even in accustomed sailors. Therefore, being sent to the crow's nest was also considered a punishment.
The crow was an essential part of the early sailors' navigation equipment. These land-lubbing fowl were carried on board to help the navigator determine where the closest land lay when the weather prevented sighting the shore visually. In cases of poor visibility, a crow was released and the navigator plotted a course that corresponded with the bird's because it invariably headed straight toward land, "as the crow flies."
The crow's cage was situated high in the main mast where the look-out stood his watch. Often, he shared this lofty perch with a crow or two since the crows' cages were kept there: hence the "crow's nest.